The mechanical properties of the infant's chest wall differ substantially from the adult's, but little is known about the control of the infant's respiratory muscles. During the past two years, we have studied how lung volume is maintained in infants and how increases induced in lung volume affect chest wall dynamics. We have shown that application of continuous distending airway pressure (CDAP) to infants results in rib cage expansion, while diaphragm configuration, inferred from abdominal diameter, changes little, and mask occlusion pressure does not change. The prolongation of expiration that occurs when CDAP is applied results in a slowing of respiratory rate associated with little change in tidal volume. We have used a flow-volume representation to demonstrate that full term infants dynamically maintain their end-expiratory volume higher than relaxation volume. In addition, we have developed improved techniques for quantification and power density spectral analysis of diaphragm, intercostal muscle and abdominal muscle surface electromyograms (EMG). These techniques will be used to investigate further the maintenance of lung volume and responses to changes in lung volume in full term and premature infants. The activity of the respiratory muscles will be characterized under conditions of increased drive, and we will describe for the first time in infants any changes in the power spectrum of the diaphragm EMG during the time course of inspiration. The interaction of the respiratory muscles will be investigated during increases in lung volume produced by changes in posture, sleep state, and applicaiion of CDAP. We hypothesize that the active Hering-Breuer response of newborn infants enables them to defend lung volume, in contrast to the adult mechanism of compensation for changes in diaphragm operational length induced by changes in lung volume.